The Crust, Mantle and Core of the Earth Text Box: Text Box: Our planet is made up of layers, the crust which
is about 40km thick and of which we live on. The mantle
is made of
magma and is semi rigid. The core
is made of iron/nickel. This came
about millions of years ago since heavier material gravitated towards
the centre and lighter material floated to the surface. The dense,
solid, inner core of iron is surrounded by a liquid-iron outer core.
The Lithosphere is the near-rigid outer shell of the Earth, including
the crust and the uppermost layer of the mantle. It is made up of many
blocks that are in motion relative to each other called TECTONIC
The lithosphere is cracked into many large interlocking pieces, which
move slowly at speeds of several centimetres each year. They are on
the top of the mantle because they are less dense than the mantle
They can move apart, move together or slide past each other. The
plates fit together or interlock which makes scientists believe that
all of the continents were joined together and due to continental
drift they have moved apart to what they are today.
Tectonic activity can create a range of features, including mountain
ranges and volcanic island arcs, as well as generating earthquakes and
There are logically three ways that plate margins
can move relative to
Moving away from one another.
This results in new oceanic crust being formed as lava fills the gap
between the plates. This is known as a constructive margin and is what
occurs at a mid oceanic ridge.
Moving towards each other.
These margins are called "destructive margins" since crust gets
destroyed as the plates collide. If two continental plates collide
then the crust ruptures and crumples up forming a mountain range such
as the Himalayas (which are forming as the Indian plate slowly crashes
into the Eurasian plate.) Alternatively if an oceanic plate collides
with a continental plate then the continental crust, being more
buoyant, rides over the top of the oceanic plate. The oceanic plate is
subducted back into the mantle, thus destroying oceanic crust, to
balance the crust being produced at the mid oceanic ridges.
This is why all oceanic crust is much younger than the continental
crust; it is constantly being recycled. Even though new oceanic crust
is always being formed, old crust is always being destroyed, and so
there is no very ancient oceanic rock around. If this didn't happen,
the world would have to be constanly expanding to make way for the
extra crust being formed!
As the oceanic plate gets pushed down into the mantle, a vast ocean
trench is formed by the drastic lowering of the sea bed. These
trenches are by far the deepest areas of the worlds' ocean and are
home to some of the planet's most extraordinary wildlife. Sometimes
some of the subducted oceanic plate, once melted into magma within the
mantle, begins to rise and push up through the continental plate on
the other side, forming volcanoes, and ultimately, a mountain range
such as the Andes (caused by the Nazca plate sinking below the South
Sliding past each other.
Tectonic plates are also able to slide in opposite directions whilst
lying next to one another. As crustal material is neither destroyed
nor created in this procedure these are known as conservative margins.
However the edges of the plates are rough and cause friction. This
means that rather than sliding smoothly past each other they tend to
jam and stick in one place until the pressure builds up to be so great
that it has to give. At that point the plates move suddenly, causing
an earthquake. For this reason the fault lines along conservative
plate margins tend to often be the most dangerous earthquake zones in
the world. For example, the San Andreas Fault forms a junction between
the North American and Pacific Plates. Although both plates are moving
northwest, the pacific plate moves faster, giving the illusion that
they are moving in opposite directions at a rate of about 6cm per
year. When sufficient pressure has built up, the pacific plate
suddenly jerks forwards, resulting in massive earthquakes in
California. If this process continues, Los Angeles will eventually end
up as an island off the Canadian coast!
SUBDUCTION is the process in which one plate is pushed downward
beneath another plate into the underlying mantle when plates move
towards each other. The plate that is denser will slide under the
thicker, less dense plate. Faulting occurs in the process. The
subduction zone is the place where two lithospheric plates come
together, one riding over the other. Most volcanoes on land occur
parallel to and inland from the boundary between the two plates. The
subducted plate usually moves in jerks, resulting in earthquakes. The
area where the subduction occurs is the subduction zone. A long,
narrow, deep depression forms in this area. It is called an oceanic
Types Of Rock
There are 3 types of rock, Metamorphic, Igneous and Sedimentary.
IGNEOUSâ€¦â€¦â€¦.Granite and Basalt
Igneous rocks are formed from molten magma or lava. The word, igneous
means "fire". All igneous rock starts deep in the earth as hot, molten
magma. If the magma cools and hardens inside the earth it is called
"intrusive" rock. These rocks cool slowly andhave large crystals. When
the magma comes out of the earth's crust through a volcano, it is
called "extrusive".If It cools off quickly, the crystals that form are
very small and if it cools slowly the crystals are larger. Molten, or
hot, liquid rock is called MAGMA when it is still inside the earth,
but once it comes out through a volcano it is called LAVA.
SEDIMENTARYâ€¦â€¦â€¦.Limestone and Chalk
These are layeredrocks .everyppe of every day, rocks are being worn
down by wind and rain. Tiny grains of dirt, sand, mud and clay are
worn off and washed into streams, rivers, lakes and oceans. When these
tiny bits of sand and dirt settle to the bottom of the water, they are
called sediment. Minerals in the water and microscopic, or very tiny
sea animals also get mixed in with the dirt and sand to form the
sediment, hence sedimentary rock contains fosils. er thousands and
millions of years we end up with a really deep pile of sediment. Due
to great weight and PRESSURE from all the stuff on top turns the
sediment on the bottom into sedimentary rock!
MEAMORPHICâ€¦â€¦â€¦.Marble and Gniss
Metamorphic rocks are formed when other kinds of rocks are changed by
great heat and pressure inside the earth. The word "metamorphic" means
CHANGED. Think of metamorphic rocks as recycled rocks. When igneous,
sedimentary or even metamorphic rocks get buried deep beneath the
surface of the earth, over millions of years the heat and Pressure
inside the earth change them into something else.They contain
distorted crystals. Limestone can be changed to marble, sandstone can
be changed into quartzite, and shale can be changed to slate. It's
just another example of how the earth is constantly changing!
Earthquakes are the Earth's natural means of releasing stress. When
the Earth's plates move against each other, stress is put on the
lithosphere. When this stress is great enough, the lithosphere breaks
or shifts. Imagine holding a pencil horizontally. If you were to apply
a force to both ends of the pencil by pushing down on them, you would
see the pencil bend. After enough force was applied, the pencil would
break in the middle, releasing the stress you have put on it. The
Earth's crust acts in the same way. As the plates move they put forces
on themselves and each other. When the force is large enough, the
crust is forced to break. When the break occurs, the stress is
released as energy which moves through the Earth in the form of
seismic waves which we feel and call an earthquake.
Volcanoes form when hot material from below rises and leaks into the
crust. This hot material, called magma, comes either from a melt of
subducted material, and which is light and buoyant after melting, or
it may come from deeper in the Interior of a planet and is light and
buoyant because it is *very* hot.
Magma, rising from lower reaches, gathers in a reservoir, in a weak
portion of the overlying rock called the magma chamber. Eventually,
but not always, the magma erupts onto the surface.
Volcanoes can be dome- or cone-shaped, depending on the kind, and the
composition of material ejected during eruptions. This material
includes molten lava, pyroclastic material, gases, and steam.
As you can see from the map the earthquakes and volcanoes occur on or
near the plate boundaries and subduction zones. As landforms,
volcanoes are formed by the deposition of the magma that flows or is
ejected, normally from one or several circular vents, as molten or
As You can see, the volcanoes and earthquakes generally occur along
the plate boundaries , or the subduction zones, this correlation is
shown on map 1.